THE FOOD OF THE PLANT 49 



It is thus proved that Hght is necessary for the 

 formation of starch in leaves. A further experiment 

 may be made to amphfy and confirm the above, or 

 may be substituted for it if preferred. 



Experiment 30 



Aim. — To find out if hght and darkness affect the 

 formation of starch in the leaf. 



Method. — Choose a potted plant that has a large 

 number of leaves. Take off one leaf ; boil it in water ; 

 then put it into a bottle of methylated spirit. Carefully 

 label the bottle with the date. Put the plant into the 

 dark. The next day take off another leaf and treat 

 this in the same way as the first. Similarly, each day 

 for a week remove one leaf. Then bring the plant into 

 the sunlight again and, in the light, continue the col- 

 lection of leaves for a second week. 



At the end of the fortnight examine all the leaves 

 for the presence of starch. 



Observations. — The first leaf gave a dark blue starch 

 reaction. In the second the reaction was fainter. After 

 that the amount of starch gradually decreased until 

 there was none remaining. 



On the eighth day, that is, the first after the plant 

 had been taken from the dark, a little starch was again 

 found. Each day after this the amount of starch in- 

 creased. 



Inferences. — A green leaf loses its starch if kept in 

 the dark and regains it when brought back again into 

 the light. 



Experiment 31 



Aim. — To find out which light-rays are most effective 

 in starch formation. 



Method. — The light-rays are made to pass through 

 coloured screens (blue or red) before they reach the 

 plant. The screens may be of red or blue glass, or 

 coloured solutions may be used. 



If the screens are to be of glass, two wooden boxes, 



D 



